a) Field of the Invention
The present invention relates to a method for the preparation of spherical melamine resin particulates having crosslinked structure, being excellent in resistance to heat, abrasion and solvent, and consisting of uniform spheres having a particle size of 10 .mu.m or less.
b) Description of the Related Art
A method for mechanically crushing crosslinked melamine resin has been known as a general method for the production of melamine resin particulates. However, the particulates obtained by such method are irregular shaped and has a broad particle size distribution. Consequently, in the case of using as an organic pigment or a filler, the particulates have a disadvantage of causing various problems and development of spherical and crosslinked melamine resin particulates having a narrow particle size distribution has been strongly desired.
Another prior art for preparing the spherical and crosslinked melamine resin particulates has been proposed in Japanese Patent Publication 32-5743 (1957). An acid catalyst such as ammonium chloride is added to an aqueous solution containing methylolmelamine and the condensation and crosslinking reaction of melamine resin is progressed by maintaining said aqueous solution at a temperature of 65.degree. to 75.degree. C. and pH of 5 to 7. A large amount of water is rapidly added to said reaction system at the moment when a transparent aqueous solution becomes turbid, and successively allowed to stand to form the spherical melamine resin particulates.
In the method, the particulate forming reaction is forced to stop in the course of the reaction. Hence, the method causes a problem that the particulate size is remarkably dependent upon the addition time of diluting water to the reaction mixture and the pH of the reaction mixture. Narrow control range of pH and temperature is inevitable for the crosslinking and particulate forming of the melamine resin and strict control is also required for the addition time of diluting water to the reaction mixture. Hence the method is quite unsatisfactory as a technique for performing continuous production with constant quality of products in industry.
Japanese Patent Publication 43-29159 (1968) discloses a method for preparing melamine resin particulates by reacting an aqueous solution containing melamine and formaldehyde or an aqueous solution containing a precondensate of melamine and formaldehyde in the presence of protective colloid at pH 6.0 to 8.0 until solid phase is formed. In the next step, the resin particulates obtained is further divided into a fraction having an average particle size of 1 .mu.m or less.
The method also requires to control the pH of the reaction system in a narrow range. Additionally, a hydrophilic high-molecular weight protective colloid used for the reaction leads to secondary aggregation of particulates formed in the reaction system. Thus the particulates obtained requires pulverization treatment, which is a problem of the method.
Japanese Patent Laid-Open Publication 50-45852 (1975) describes a method for the preparation of cured resin particulates. To 1 mole of a mixture composed of 100 to 50 parts by weight of benzoguanamine and 0 to 50 parts by weight of melamine, 1.2 to 3.5 moles of formaldehyde is added. The pH of the resulting aqueous solution is adjusted in the range of 5 to 10, thereby a soluble resin having a methanol miscibility of 0 to 150% is obtained. The reaction mixture is then poured with stirring into an aqueous solution of protective colloid to give an emulsion of a soluble and fusible resin. A curing catalyst is added to the emulsion and the resulting mixture is maintained in the temperature of 40.degree. to 60.degree. C. for at least an hour and successively cured in the temperature range of 60.degree. to 200.degree. C. under atmospheric or increased pressure. The cured resin thus obtained is separated, washed, dried and subjected to deaggregation treatment.
The method involves various problems. For example, a large amount of expensive benzoguanamine is used in combination with melamine and hence leads to a high cost of the product. Complex procedures are also required for the production of resin particulates. Further, the hydrophilic high molecular weight compound which is used as a protective colloid in the production step of the resin leads to secondary aggregation of the particulates in the curing and particulate forming reaction of melamine and requires grinding treatment of the resin particulates obtained.
Japanese Patent Laid-Open Publication 62-10126 (1987) proposes a method for the preparation of spherical and cured melamine resin particulates. The particulates are prepared by reacting melamine with aldehyde compounds in an aqueous medium in the presence of a basic catalyst and inorganic salts which are insoluble in water. The particulates are partly or wholly covered with substantially water-insoluble inorganic salts and have a particle size of 500 .mu.m or less.
However, the spherical and cured melamine resin particulates of the method are obtained by covering the particulate surface with water-insoluble inorganic salts and provided with water resistance and hardness by the effect of inorganic salts. Consequently, the particulates obtained by the method have disadvantage that the resistance to abrasion and solvents is inferior to those of crosslinked melamine resin particulates.